[PATCH] sched: fix/optimise calculation of weight-inverse

[Stephan Baerwolf]

If the inverse loadweight should be zero, function "calc_delta_mine"
calculates the inverse of "lw->weight" (in 32bit integer ops).

This calculation is actually a little bit impure (because it is
inverting something around "lw-weight"+1), especially when
"lw->weight" becomes smaller. (This could explain some aritmetical
issues for small shares...)

The correct inverse would be 1/lw->weight multiplied by
"WMULT_CONST" for fixcomma-scaling it into integers.
(So WMULT_CONST/lw->weight ...)

For safety it is also important to check if division by zero
could happen...

The old, impure algorithm took two divisions for inverting lw->weight,
the new, more exact one only takes one and an additional unlikely-if.

Signed-off-by: Stephan Baerwolf <stephan.baerwolf@xxxxxxxxxxxxx>
---
 kernel/sched.c |   12 +++++++++---
 1 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 312f8b9..bb55996 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1307,15 +1307,21 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight,
 {
 	u64 tmp;
 
+	tmp = (u64)delta_exec * weight;
+
+	// actually we would have to trap - division by zero - but we stay and pretend the limit of the operation...
+	if (unlikely(lw->weight == 0)) {
+		if (unlikely(tmp == ((u64)0))) return (unsigned long)0;
+		else return (unsigned long)LONG_MAX;
+	}
+
 	if (!lw->inv_weight) {
 		if (BITS_PER_LONG > 32 && unlikely(lw->weight >= WMULT_CONST))
 			lw->inv_weight = 1;
 		else
-			lw->inv_weight = 1 + (WMULT_CONST-lw->weight/2)
-				/ (lw->weight+1);
+			lw->inv_weight = WMULT_CONST / lw->weight;
 	}
 
-	tmp = (u64)delta_exec * weight;
 	/*
 	 * Check whether we'd overflow the 64-bit multiplication:
 	 */
-- 
1.7.3.4


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